The present invention relates generally to optical interferometry, and more particularly to a method of processing interferometric data from an object having multiple optical interfaces.
Optical interferometry has been used to measure physical properties of an object. For example, commonly assigned U.S. Pat. No. 5,659,392, issued Aug. 19, 1997 to Marcus et al., entitled xe2x80x9cAssociated Dual Interferometric Measurement Apparatus for Determining a Physical Property of an Object,xe2x80x9d and U.S. Pat. No. 5,596,409, issued Jan. 21, 1997 to Marcus et al., entitled xe2x80x9cAssociated Dual Interferometric Measurement Method for Determining a Physical Property of an Object,xe2x80x9d relate to an associated dual interferometric apparatus and method for measuring physical properties of an object, such as thickness, group index of refraction, and distance to a surface. U.S. Pat. No. 5,757,485, issued May 26, 1998 to Marcus et al., entitled xe2x80x9cDigital Camera Image Sensor Positioning Method Including a Non-Coherent Interferometer,xe2x80x9d and U.S. Pat. No. 5,757,486, issued May 26, 1998 to Marcus et al., entitled xe2x80x9cDigital Camera Image Sensor Positioning Apparatus Including a Non-Coherent Light Interferometer,xe2x80x9d disclose a non-coherent light interferometer in communication with an optical probe apparatus for determining a position of an image sensor within a digital camera relative to a reference surface.
While the methods disclosed by these references have achieved certain degrees of success in their particular applications, a need continues to exist for an automated method for processing interferometric data, particularly when additional, superfluous interferometric data is obtained. More particularly, when processing interferometric data, automated methods are needed to make a determination regarding which interferometric data is relevant and which data is superfluous. A need also continues to exist for an automated method for processing interferometric data when some interferometric data is missing or hidden in the measurement noise. Further, when the interferometric data corresponds with a multi-component physical entity, such as reflections from a plurality of optical interfaces, reflections between the various interfaces may result in extra peaks in the interferogram. As an example, a double reflection from a pair of adjacent interfaces would cause an extra peak to occur at twice the actual optical thickness of the layer defined by the adjacent interfaces. To properly analyze the data, these extra peaks need to be removed from the data. This process is time consuming and tedious. There is a need therefore for a rapid automated method for removing extra peaks from interferograms prior to analysis.
The above noted need is met according to the present invention by providing a method of processing interferometric data relating to a sample having multiple reflective surfaces, that includes the steps of: specifying acceptance ranges for peaks of interest in the interferogram, based on a knowledge of the sample; determining which peaks in the interferogram fall within the acceptance ranges; identifying which peaks within the acceptance ranges represent multiple reflections and eliminating these peaks from the interferometric data; identifying acceptance ranges which do not contain peaks; and employing other peaks in the interferogram to calculate the locations of peaks in the acceptance ranges that do not contain peaks.
The present invention provides an automated method for processing interferometric data, particularly when superfluous interferometric data is obtained. The present invention further provides for the processing of interferometric data when some interferometric data is missing or hidden in the measurement noise. The method provides for a correspondence between the interferometric data and a physical entity to which the data corresponds.